1. Technical Field
The present invention relates to wireless radio systems and, more particularly, frequency synthesizers for use in radio front end circuitry to down-convert RF signals.
2. Related Art
The demand for high performance universal frequency synthesizers is growing with the increasing performance and integration requirements of wireless radio frequency (RF) systems, such as cellular telephony and FM radio systems. Phase locked loop (PLL) frequency synthesis is a popular indirect frequency synthesis method for high performance applications due to its agility and the ability of synthesizing frequencies over wide bandwidths with narrow channel spacing. However, PLL synthesizer design still remains a challenging aspect of RF system design, because of the stringent requirements typically imposed on frequency synthesizers. For example, frequency synthesizers are typically required to be defined with an output frequency accuracy on the order of a few parts per million (PPM). Furthermore, in most cases, the output frequency must also be capable of being varied in small precise steps, such as a few hundred kilo-hertz (kHz), corresponding to the RF channel spacing.
In addition to accuracy and channel spacing, other aspects of PLL frequency synthesizers influence the performance of a receiver, such as phase noise, reference spurs and lock time. In radio receivers, if the phase noise produced by the frequency synthesizer mixes with nearby interferers that are then converted onto the desired channel, the signal-to-noise ratio of the received signal can be adversely affected. In addition, reference spurs may cause the receiver to down-convert undesired interferers. Furthermore, the lock time required in typical RF systems varies from a few milliseconds (ms.) to a few tens of microseconds (us.). As used herein, the term “lock time” refers to an indication of how fast a new frequency is established when the RF receiver commands a change in the channel.
Frequency synthesizers typically include a precise crystal oscillator (X-TAL) providing a reference frequency, a phase and frequency detector (PFD), a charge pump (CP), a lowpass loop filter (LPF), a voltage controlled oscillator (VCO), and one or more divider blocks in the feedback path that each divide the incoming signal by some integer of either fixed or on-the-fly programmable value to produce a feedback signal. Ideally, the charge transferred into the LPF from the CP is proportional to the phase difference between reference and feedback signals. In practice, however, the characteristics of the PFD/CP combination do not provide a completely linear transfer curve. Loop filter leakage currents in combination with non-uniform phase error sampling gives rise to considerable “reference spurs” (i.e., periodic modulations of the VCO generating tones around the RF carrier) and elevated in-band noise. Thus, the VCO output, i.e., the RF carrier, contains sidebands corresponding to such “feed-through” of the reference frequency.
In many applications, reference feed-through may have a detrimental effect on receiver performance if not appropriately limited. For example, reference feed-through may mix with a high-powered out-of-band interferer and thereby degrade the in-band signal-to-noise (SNR) ratio. Even worse, if the reference spur is generated directly within the signal band, the down-converted radio signal contains copies of the desired signal at frequency offsets corresponding to the reference frequency.
Therefore, a need exists for a frequency synthesizer design for use in radio receivers that minimizes reference feed-through.